The present invention relates to a method for conducting the water-gas shift reaction with platinum group metal-based water gas shift catalysts that include a methane suppressing component selected from oxides of tin or gallium. The inclusion of the methane suppressing component in the catalyst composition prevents or reduces the formation of methane that is often observed on treatment of carbon monoxide in hydrogen-containing gas stream. The present invention also relates to methods for the use of these catalysts for generating hydrogen by reaction of carbon monoxide (CO) and steam (gaseous H2O), and in particular to generating hydrogen from a gas stream comprising hydrogen, water, and carbon monoxide. The catalysts and methods of the invention are useful, for example, in generating hydrogen in the gas stream supplied to fuel cells, particularly to proton exchange membrane (PEM) fuel cells.
Fuel cells directly convert chemical energy into electricity thereby eliminating the mechanical process steps that limit thermodynamic efficiency, and have been proposed as a power source for many applications. The fuel cell can be two to three times as efficient as the internal combustion engine with little, if any, emission of primary pollutants such as carbon monoxide, hydrocarbons and nitric oxides. Fuel cell-powered vehicles which reform hydrocarbons to power the fuel cell generate less carbon dioxide (green house gas) and have enhanced fuel efficiency.
Fuel cells, including PEM fuel cells [also called solid polymer electrolyte or (SPE) fuel cells], generate electrical power in a chemical reaction between a reducing agent (hydrogen) and an oxidizing agent (oxygen) which are fed to the fuel cells. A PEM fuel cell includes an anode and a cathode separated by a membrane which is usually an ion exchange resin membrane. The anode and cathode electrodes are typically constructed from finely divided carbon particles, catalytic particles supported on the carbon particles and proton conductive resin intermingled with the catalytic and carbon particles. In typical PEM fuel cell operation, hydrogen gas is electrolytically oxidized to hydrogen ions at the anode composed of platinum reaction catalysts deposited on a conductive carbon electrode. The protons pass through the ion exchange resin membrane, which can be a fluoropolymer of sulfonic acid called a proton exchange membrane. H2O is produced when protons then combine with oxygen that has been electrolytically reduced at the cathode. The electrons flow through an external circuit in this process to do work, creating an electrical potential across the electrodes. Examples of membrane electrode assemblies and fuel cells are described in U.S. Pat. No. 5,272,017.
Fuel cell processors (also known as fuel cell reformers) supply a hydrogen-containing gas stream to the fuel cell. Fuel cell processors include reactors that steam reform hydrocarbon feedstocks (e.g., natural gas, LPG) and hydrocarbon derivatives (e.g., alcohols) to produce a process stream enriched in hydrogen. Other by-products from the steam reforming of hydrocarbon include carbon monoxide and carbon dioxide. For example, methane is converted to hydrogen, carbon monoxide and carbon dioxide by the two reactions below:
CH4+H2Oxe2x86x923H2+CO
CH4+2H2Oxe2x86x924H2+CO2
The resulting gas is then reacted in the water-gas shift reactor where the process stream is further enriched in hydrogen by reaction of carbon monoxide with steam in the water-gas shift reaction:
CO+H2OCO2+H2
In fuel cell processors, the reaction is often conducted in two stages for purposes of heat management and to minimize the outlet CO concentration. The first of two stages is optimized for reaction at higher temperatures (about 350xc2x0 C.) and is typically conducted using catalysts based on combinations of iron oxide with chromia. The second stage is conducted at lower temperatures (about 200xc2x0 C.) and is typically conducted using catalysts based on mixtures of copper and zinc materials.
Other catalysts that can be used to conduct the water-gas shift reaction include platinum-based catalysts such as platinum on an alumina support or platinum on a cerium oxide support. While effective at producing hydrogen using the water-gas shift reaction when operated at temperatures above about 300xc2x0 C., water-gas shift reaction catalysts also cause the formation of methane (CH4) by catalyzing the reaction of CO with hydrogen as shown below:
CO+3H2xe2x86x92CH4+H2O.
This undesired side reaction sacrifices three moles of hydrogen for each mole of carbon monoxide converted to methane. Methanation can also occur under these conditions with carbon dioxide according to the equation shown below:
CO2+4H2xe2x86x92CH4+2H2O.
In this side reaction four moles of hydrogen are consumed for each mole of carbon dioxide converted to methane. The production of methane during the water gas shift reaction (referred to herein as xe2x80x9cmethanationxe2x80x9d) is a side reaction that consumes hydrogen gas in an exothermic reaction to ultimately reduce the hydrogen yield from the water gas shift reaction. Moreover, the methanation reactions accelerate with increasing catalyst bed temperatures. This property presents a liability, as the exothermic reaction can result in a runaway reaction with carbon dioxide, in addition to carbon monoxide, being methanated. Major hydrogen loss can occur and the catalyst can be damaged by high temperatures. In addition, methane is a greenhouse gas. The fuel cell is advertised as an emission-free energy producer, and release of methane is undesirable. Methane is difficult to combust during normal operating conditions of the fuel cell, so producing an appreciable quantity of methane is environmentally unfavorable.
In one embodiment, the invention relates to a method of producing hydrogen from an input gas stream containing carbon monoxide and steam. The method includes contacting the input gas stream with a catalyst that contains: an inorganic oxide support; a platinum group metal dispersed on the inorganic oxide support; and a methane suppressing component. The methane suppressing component is selected from the group consisting of oxides of tin, oxides of gallium and combinations thereof. The methane suppressing component is also dispersed on the inorganic oxide support.
The platinum group metal used in the method is selected from the group consisting of platinum, palladium, rhodium, ruthenium and iridium. Preferably, there is from about 0.1 to 10 wt. % of the platinum group metal dispersed on the inorganic oxide support. A preferred platinum group metal is platinum.
The inorganic oxide support of the catalyst is selected from the group consisting of alumina, zirconia, titania, silica, oxides of rare earth metals and combinations thereof. The rare earth metals are selected from the group consisting of the lanthanides and yttrium.
One preferred inorganic oxide support is formed from a composite of zirconia and oxides of one or more rare earth metals. A particularly preferred support is formed from a composite of cerium oxide and zirconia.
Another preferred inorganic oxide support used in the method contains alumina impregnated with one or more oxides of rare earth metals selected from the group consisting of the lanthanides and yttrium. More preferably, the method uses a catalyst that has a support composed of alumina impregnated with both zirconia and one or more oxides of rare earth metals.
The catalyst used in the method is preferably in the form of a washcoat composition deposited on a substrate. Preferably, the substrate is a honeycomb monolith.
In a preferred embodiment of the method, the input gas stream is contacted with a catalyst having: a cerium oxide-zirconia support; about 0.01 to 10 wt. % of platinum dispersed on the cerium oxide-zirconia support; and about 0.01 to 5 wt % of oxides of tin dispersed on the cerium oxide-zirconia support. In particularly preferred embodiments, there is about 1 to 5 wt. % platinum dispersed on the cerium oxide-zirconia support; and about 0.1 to 2 wt. % of oxides of tin dispersed on the cerium oxide-zirconia support.
In another aspect, the invention relates to an apparatus for supplying hydrogen to a fuel cell with a hydrocarbon reformer reactor, a selective carbon monoxide oxidation reactor and a water-gas shift reactor having a water-gas shift catalyst. In the apparatus, the water-gas shift catalyst has an inorganic oxide support and a methane suppressing component selected from the group consisting of oxides of gallium, oxides of tin and combinations thereof. The methane suppressing component is also dispersed on the inorganic oxide support. The hydrocarbon reformer reactor is upstream and in train with the water-gas shift reactor, and the preferential oxidation catalyst is downstream and in train with the water-gas shift reactor. Preferably, the support is a cerium oxide-zirconia composite.
In another aspect, the invention relates to a method of preparing a catalyst having an inorganic oxide support, a platinum group metal dispersed on the inorganic oxide support, and oxides of tin dispersed on the inorganic oxide support. The method includes the steps of:
a) suspending the inorganic oxide support in an aqueous solvent at a pH below 3 to form a first aqueous suspension;
b) adding tin powder to the first aqueous suspension to form a second aqueous suspension;
c) adjusting the pH of the second suspension to a pH of at least 8 by adding base;
d) collecting the suspended particles from the product of step c; and
e) drying and calcining the precipitate.
In a preferred embodiment of the method, the first aqueous suspension contains a platinum group metal salt, preferably a platinum salt. In this method, preferred inorganic oxide supports contain rare earth metals such as cerium oxide-zirconia composites.